The present invention relates to a thermal printer for printing on a thermal roll sheet with a thermal head while feeding the thermal roll sheet. The present invention also relates to a cover open mechanism applied to such a thermal printer.
Generally, this type of thermal printer has a simple structure and is thus advantageous in miniaturization, cost reduction, and facilitation of maintenance. Accordingly, this type of thermal printer is used in various devices or systems which require miniaturization, cost reduction, and facilitation of maintenance, such as POS terminals for issuing receipts and automatic dispensers for issuing tickets or coupons.
This type of thermal printer has a base frame, a cover frame capable of opening and closing with respect to the base frame, a platen mounted on the cover frame, and a linear thermal head mounted on the base frame. The thermal head is operable to press the platen in a radial direction of the platen.
The thermal head is mounted on a head bracket, which is attached to the base frame in a movable state. Further, a cover operation member for opening and closing the cover frame is attached to the base frame in a movable state. The thermal head is biased together with the head bracket by coil springs or the like so as to press the platen when the cover frame is closed. The conventional thermal head is biased together with the head bracket so as to press the platen in a radial direction of the platen that is in parallel to a direction in which the cover frame is to be opened.
The thermal printer having the above structure is used in the following manner.
First, the cover operation member, which is attached to the base frame in a movable state, is operated so as to unlatch the cover frame. When the cover frame is unlatched, the cover frame is automatically pushed and opened because the platen is pushed by a biasing force of the thermal head. Then, a thermal roll sheet is set on a set position for the thermal roll sheet, which is exposed when the cover frame is opened. An end of the thermal roll sheet is manually fed by a proper length and placed on the thermal head. Subsequently, the cover frame is closed. The cover frame is held in the closed state by the cover operation member.
When a printing command is inputted after power is turned on, the thermal head prints on the sheet between the platen and the thermal head while the platen is rotated to feed the thermal roll sheet.
The printed sheet, which passes between the platen and the thermal head, is guided along a quadrant path formed in the cover frame so as to turn at 90 degrees and then discharged from a discharge port formed in a top surface of the cover frame. The quadrant path has a relatively large curvature so as to prevent a frictional resistance from increasing to cause a paper jam when a paper passes through the quadrant path. Accordingly, the quadrant path has a relatively large size.
The inventors have developed a structure for reducing the size of a conventional thermal printer by eliminating a quadrant path as described above. In this structure, a thermal head is biased together with a head bracket so as to press a platen in a radial direction of the platen that is substantially perpendicular to a direction in which a cover frame is to be opened. Specifically, a printed sheet which passes between the platen and the thermal head, is directed toward a discharge port formed in a top surface of the cover frame without changing from a direction in which the printed sheet passes between the platen and the thermal head. Thus, this improved structure can eliminate a space required for a quadrant path in the conventional structure and can thus reduce the size of a thermal printer.
However, in the aforementioned structure, even if the cover frame is unlatched by operating the cover operation member in order to set a thermal roll sheet on the thermal printer, the cover frame is not automatically opened for the following reasons. Since the thermal head is biased so as to press the platen in a direction substantially perpendicular to a direction in which the cover frame is to be opened, the cover frame (the platen mounted on the cover frame) is not biased in the direction in which the cover frame is to be opened by the thermal head. Additionally, since the platen is pressed in the aforementioned direction by the thermal head, the cover frame is held so as not to be opened.
Accordingly, in order to open the cover frame, a user has to operate the cover operation member by one hand to unlatch the cover frame and hold the cover frame by the other hand to lift the cover frame. Thus, it is not easy to use the improved structure.
The following structure may be considered in order to solve the above problem. A spring is provided for biasing the cover frame in a direction in which the cover frame is to be opened. When the cover operation member is operated, the cover frame is unlatched while the pressing of the platen by the thermal head is released. The free cover frame is automatically opened by a force of the spring.
However, according to experiments conducted by the inventors, a considerably large operating force was required during an overall operation after the operation of the cover operation member was started. Thus, it was found that the usability was degraded. This was caused by the fact that a biasing force applied to the thermal head was as large as about 1.5 kgf because of specifications of the thermal printer.